Loud-speaker and circuit therefor



Dec. 9, 1930' c, FARRAND 1,784,486

LOUD SPEAKER AND CIRCUIT THEREFOR Filed Feb. 26, 1929 odylNvEN np aw,

ATTORNEYS,

Patented Dec. 9, 1930 UNITED STATES PATENT OFFICE CLAIR L. FARRAND, OFFOREST HILLS, NEW YORK, ASSIGNOR T0 FARRAND INDUCTOR CORPORATION, OFJERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWARE LOUD-SPEAKER CIRCUITTHEREFOR Application filed February 26, 1929. Serial No. 342,899.

This invention relates to radio loudspeakers and to the circuits ofwhich they form a part.

In radio receiving apparatus it is customary to utilize some device toprevent the steady direct current in the amplifier from passing throughthe loudspeaker windings. Unless the effect of the steady direct currentis eliminated it exerts a constant biasing force on the armature of theloudspeaker motor causing an objectionable displacement of it. Perhapsthe most commonly used devices for accomplishing this are the so-calledoutput transformer and the output filter. \Vhen one of these devices isinterposed between the amplifier and the loudspeaker it has the effectof passing the fluctuating speech currents to the loudspeaker motor butit does not pass on to the loudspeaker the steady direct current.

The push-pull type of amplifier, now so commonly used, has either apush-pull output transformer or a push-pull output filter, and in eithercase prevents the steady direct current from reachin the loudspeaker.

One object of'thls invention is to provide a loudspeaker whose motor isso constructed and so connected with the amplifier that the steadydirect current is not prevented from passing to the loudspeaker but isallowed to traverse the loudspeaker windings in such a. Way that it doesno harm, and in some cases may be utilized to advantage.

A further object is to provide a loudspeaker whose construction adaptsit for connection in an amplifying circuit to produce the abovementionedresults.

Ali embodiment of the invention is illustrated in the accompanyingdrawing which diagrammatically represents the improved loudspeakerconnected in circuit with a portion of apush-pull amplifier in a mannerthat will produce the results contemplated by the invention. 4

In the drawing the numeral 10 represents an input push-pull transformerhaving a primary coil 11 connected in circuit with the otherinstrumentalities of the radio receiver in the customary and well-knownway. The secondary 12 of the input transformer is connected at one endwith the grid 13 of a threeelectrode vacuum tube 15 and at its other endwith the grid 14 of a similar vacuum tube 16., An intermediate point 12of the transformer secondary winding is connected by means of aconductor 9 to a circuit which includes the two filaments 17 and 18 ofthe vacuum tubes and a filament battery 19. A grid bias battery 19' maybe inserted in the lead 9. A conductor 20' extends from the plate 20 oftube 15 to a,

point 28 and a similar conductor 21 extends from plate 21 of the tube 16to a point 29. A B battery 24 is connected to the common filamentcircuit 1718 and to a conductor 24' which extends to a point 30.

The arrangement so far described is the well-known push-pull amplifierup to but not including the usual output push-pull transformer. Theusual push-pull amplifier has an output push-pull transformer connectedatthe points28, 29 and 30 as follows: One end of the primary winding ofthe transformer is connected at 28 and the other end at 29. A midpointof the primary winding is connected to the lead 2-1 at the point 30.'The secondary of the output transformer is connected directly to theloudspeaker motor. In place of an output push-pull transformer an outputpushpull filter arrangement'is sometimes employed connected at the threepoints 28, 29 and 30 in a well-known manner. The customary outputpush-pull transformer, or output push-pull filter arrangement preventsthe steady direct current from the B battery 24 from reaching theloudspeaker but the fluctuating speech currents are passed on to theloudspeaker in a manner well understood in the art.

According to this invention, the output push-pull transformer. or theoutput pushpull filter arrangement usually connected at the points 28,29 and 30. is omitted altogether and the loudspeaker is so constructedthat the windings of its motor may be directly connected at the points28, 29 and 30 in such a way that the steady direct current is allowed topass through the motor windings but'in such a way that it does no harmand in some cases may be utilized to advantage. This is accomplished, inone form of the invention, by utilizing a loudspeaker, constructed andconnected in circuit as follows: The loudthe windings 39 and 40. Thefluctuating currents are of un ual value in the windings 39 and 40 and pace;

speaker motor has a source of permanent field flux such as a magnet, aportion only of which is represented at 8. The legs of the magnet areattached to two pole pieces 31 and 32, each of which is bifurcated so asto form a pair of poles 33 and 34 on the pole piece 31 and poles 35 and36 on the pole p ece 32. The poles 33 and 34 are of like polarity andthe pole pieces 35 and 36 are of like polarity. An armature 37 moves ina path parallel with the polar faces and is connected with a suitable'diaphragm such as the cone 38. The arrangement is such that a relativeincrease of flux at one end of the armature will move it longitudinallyin one direction, whereas a relative increase in flux at the other endof the armature will move it longitudinally in the other direction. 'Acoil 39 associated with one of the poles of the pole piece 31 isconnected to the points 28 and 30, while a coil 40 on the diagonallyopposite pole of pole piece 32 is connected to the points 30 and 29.

It will now be seen that due to this arrangement the steady directcurrent from the battery 24 divides at point 30 and one portion passesthrough the coil 39 to the point 28 and then through the remainder ofthe plate circuit of the tube 15. The other portion of the steady directcurrent passes from the point 30 through the coil 40 to the point 29 andthen throu h the remainder of the plate circuit of the tu 16. Whatevereffect on the armature is produced by the steady direct current passingthrough the coil 39 is ofi'set or counteracted by a similar effectproduced by the steady direct current of equal value flowing through thecoil 40. In other words these effects add to zero. If the steady directcurrent tends to increase the flux in the pole 33 it will do likewiseand to the same extent in the pole 36 and there will, therefore, be nomovement of the armature. .If the steady di- .rect current in the coil39 tends to decrease the flux in the pole 33,-the steady direct currentin the coil 40 will likewise decrease the flux in the pole 36 to thesame extent and there will be no movement of the armature. In eithercase the steady direct current in the coils 39 and 40 produces like fluxchanges in their poles. The coils 39 and 40 are preferably so wound thatthe steady direct current passing through them tends to increase theflux through their respective poles rather than decrease it. Thestrength of the field is thereby increased because "the permanent orsteady flux of the field has added to it the flux produced by the steadydirect current in unlike flux changes at the poles 33 and 36'.

That is, as'the fluctuating current increases in the coil 39 itdecreases in the coil 40, and

vice versa, so that the flux changes are unlike in that the flux isincreased in one pole while it is decreased in the other. Y The effectsproduced by the fluctuating speech currents passing through the windings39 and 40 therefore do not add to zero, as in the case of the steadydirect current, but are efi'ective in causing rcciprocation of thearmature.

\Vhile the drawing shows a coil on one pole only of the pole piece 31and on one pole only of the pole piece 32, it is apparent that the sameresults described above may be obtained when all of the poles areprovided with coils if the coils are wound in the proper direction andproperly connected.

W'hen the windings of a loudspeaker motor are connected in a circuit ofthe character of the one described in the manner above set forth, thecustomary push-pull output transformer or push-pull output filter isentirely eliminated thus saving this expense. Moreover, the lossesincidental to the use of these instrumentalities are'also eliminated.

It will now be seen that the invention eliminates the necessity of usingany device for points 28, 29 and 30 so that it may be sold as a separatearticle of manufacture in readiness to be connected to a circuit havingthe characteristics of the one shown in the drawmg.

While the principle of the invention has been illustrated by showing aloudspeaker motor connected with a portion of the usual push-pullamplifier circuit the invention also contemplates any output circuitwhich willv divide or split the steady direct current and distribute itequally to the loudspeaker motor windings and which at the same timewill properly produce the fluctuating speech currents in the motorwindings. The invention also contemplates the use of-loi speaker motorsdiffering in construction trated and described so long coils are adaptedto be properly connected with the amplifier circuit and so long as thegeneral construction of the motor is such as'to bring about theefiects'herein described,

I claim: a

1. The combination ofa loudspeaker having a motor comprising at leasttwo poles each vhaving a speecliboil and anarmature between the polesall arran ed so thatequal currents passing through t ecoils will produce like flux changes at the poles'and result unequal currents passingthrough the coils will produce unlike flux changes at the poles whichresult in a movement of the armature, an electric circuit for each ofsaid coils, said circuits having a common path, a source of directcurrent in said common path whereby steady direct current from saidsource divides and flows through both of said coils to produce like'fluxchanges at said poles resulting in no movement of the armature, andmeans in said circuits for producing fluctuating currents of unequalvalue in the speech coils which result inunlike flux changes at thepoles and a consequent movement of the armature.

2. The combination of a loudspeaker having a motor comprising at leasttwo poles each having a speech coil and an armature between the polesall arranged so that equal currents passing through the coils willproduce like flux changes at the poles and result in no movement of thearmature and so that unequal currents passing through the coils willproduce unlike flux changes at the poles which result in a movement ofthe armature, an electric circuit for each of said coils including asource of direct current such that the steady direct current flowingthrough both of said coils produces like flux changes at said polesresulting in no movement of the armature, and means in said circuits forproducing fluctuating currents of unequal value in the speech coilswhich result in unlike flux changes at the poles and a consequentmovement of the armature.

3. A combination as set forth in claim 2 in which the poles of the motorare associated with a source of permanent flux.

4. A combination as set forth in claim 2 in which the poles of the motorare associated with a source of permanent flux and in which the speechcoils are so wound as to cause the steady direct current passing throughthem to add to and increase the permanent flux.

5. The combination with a push-pull amplifier stage comprising an inputpush-pull transformer havin primary and secondary windings, a pair 0vacuum tubes each hav ing a filamentggrid and plate, the grids beingrespectively connected with the ends of the transformer secondary, afilament circuit including the two filaments of the tubes, a connectionbetween the filament circuit and an intermediate point of thetransformer secondary, a source of direct current connected to saidfilament circuit, and leads extending from the plates of said tubes, ofa loudspeaker having a motor comprising at least two poles provided withspeech coils, an armature arranged between the poles, one of said speechcoils being connected to said direct current source and to one of saidleads and the other of said speech coils being connected also to saidsource of direct current and to the other of said leads, said speechcoils being wound to produce like flux changes in their poles as aresult of the steady direct current from said source which divides andpasses through said speech coils in 7 equal amounts.

6. The combination with a thermionic amplifier comprising a pair ofvacuum tubes aving the grid circuits thereof connected in push-pullrelation to an input circuit, of a loud speaker unit including means forproducing a permanent magnetic flux adapted to maintain in dynamicequilibrium an armature aflixed to an acoustical unit, a pair ofsignal-current coils for actuating said armature and situatedrespectively at opposite ends thereof, and means so connecting saidcoils individually in the anode circuits of said tubes, respectivel thatthe fluxes due to the space currents flbwing therein in each case aidsaid permanent flux, whereas signal waves repeated by said am lifiertubes produce additive actuating e ects upon said armature.

In testimony whereof I aflix my signature.

CLAIR L. FARRAND.

